On the microstructure and crystallography of directionally solidified MoSi2-Mo5Si3 eutectics

Directionally solidified MoSi2-Mo5Si3 eutectic rods were produced by the Czochralski method. A script lamellar microstructure was produced and an inverse square root dependence upon the pull rate (39--210 mm/h) was observed for the lamellar spacing. The eutectic rods were textured with [001] of Mo5Si3 and <110> of MoSi2 parallel to the rod axis and an orientation relationship consisting of [110]MoSi2||[110]Mo5Si3 and (1 bar 10)MoSi2||(002)Mo5Si3 was present in all of the samples. The eutectic grows with a lamellar plate morphology inclined at 15 degree relative to the rod axis and four growth variants are possible. A script lamellar microstructure, as viewed in the transverse plane, was produced by periodic branching of the Mo5Si3 lamellae and this branching was not associated with any change in crystallography. Addition of 0.35 at.% erbium produced a more fibrous microstructure and an erbium-rich compound, Er2Mo3Si4 with space group 21/c, formed congruently with the Mo5Si3-MoSi2 during solidification. A ledge-terrace growth mechanism was proposed to explain the observed microstructures